The present invention relates to compounds that are useful for surface modification of metallic particles, such as metallic flake pigments. In particular, the invention concerns decorative coatings formed from aqueous compositions containing metallic flake pigments that are surface modified with such compounds.
The use of metallic flake pigments, such as aluminum flake pigments, in decorative coatings to give the coating a metallic effect is widespread. The metallic effect is particularly popular with customers in the automotive market, and metallic colors form a large share of automotive paints.
Automotive coatings may utilize a single, uniformly pigmented layer. Alternatively, they may have two distinct layers--a lower, highly pigmented layer and an upper layer with little or no pigmentation. The two-layer coating is known in the industry as basecoat/clearcoat or color plus clear coat. Basecoat/clearcoat coatings impart a high level of gloss and depth of color that result in a particularly appealing look. Metallic flake pigments may be incorporated to give the coating a metallic effect.
Concerns over organic solvent emissions during application of paints have resulted in the introduction of waterborne automotive paints, especially waterborne basecoat paints. The new waterborne technology has the disadvantage of using a medium corrosive to the popular metallic flake pigments. When a paint with oxidized metallic flake pigments is coated onto a substrate, the coating shows discoloration and diminished metallic effect. Furthermore, oxidation of the metallic surfaces by the water results in the evolution of hydrogen gas, which may accumulate in storage of the paint.
Considerable work has been done in the industry to try to protect the metal surfaces from the water. The metallic flake may be treated with inorganic reagents, such as chromates, according to the process of Kondis (U.S. Pat. No. 4,693,754); or the flake may be first encapsulated with a silica coating and then treated with a carboxylic chromic chloride as disclosed by Batzar (U.S. Pat. No. 3,954,496). However, these treatments are known to be detrimental to the appearance of the flake in a coating, either from discoloration or decreased luster. The toxicity of the inorganic reagents used in such treatments is also a concern.
Organic phosphate or phosphite treatments have been used, including the simple phosphate esters of Williams, Jr. et al. (U.S. Pat. No. 4,565,716), such as mixtures of mono- and di-phosphates of monoalcohols like octylphenol, ethylene glycol monobutyl ether, or octanol; and polymeric esters like those disclosed by Frangou (U.S. Pat. No. 4,675,358). However, at the levels required to give protection comparable to the inorganic treatments, the coating may exhibit loss of adhesion to other layers or a cohesive failure within the coating layer.
A method of coating an aluminum flake with a monoethylenically unsaturated silane, said silane then being reacted with acrylic monomers, is disclosed by Turner (U.S. Pat. No. 4,213,886). However, this method does not provide for a well-dispersed flake pigment in the aqueous paint composition. The poor dispersion of the flake pigment diminishes the metallic effect in the coating prepared therefrom.
It has been discovered that metallic flake pigments that are surface modified with the compounds of the present invention are particularly resistant to oxidation in waterborne paints with minimal discoloration or diminution of the metallic effect in the coating. It has also been discovered metallic flake pigments surface modified according to the present invention form superior dispersions in water in comparison to metallic flake pigments using previously known surface treatments. This superior dispersion in the waterborne paint composition results in a coating with an enhanced metallic effect. Moreover, when the compounds of the present invention are used at levels necessary to prevent oxidation of the metallic pigment, the coating does not exhibit any loss of adhesion to other layers or any cohesive failure within the coating layer.